M-16 FrankenSTEN: Creating a Monster - A How-to Guide

By Will Dabbs, MD

The Good Ole Days

Back in the good ole days, before the 1986 machinegun ban, Turbo Publishing produced a monthly magazine called Firepower. Printed therein were articles with titles like “Building a STEN,” “Convert Your Own AK,” and “M1 to M2 Carbine Conversion.” They featured a different full-auto conversion almost every month and my dog-eared collection of back issues is still readable. Sadly, times have changed quite a bit and private citizens are no longer trusted to register and build machineguns at home. All is not lost, however, for the average American firearms hobbyist with a little creativity and a lot of time.

Untapped Potential

Mark III STEN kits are dirt-cheap. With gun parts suppliers practically giving them away I felt that surely there was something I could legally build out of one. A STEN submachine gun would be the obvious choice but as I am a physician and not a Class II firearms manufacturer I value my freedom too highly to consider that option. After several weeks of cerebrating on the subject I awoke one morning with an epiphany and, with this as a basis, embarked on a personal quest to build a FrankenSTEN.

Gun building is a potentially hazardous undertaking and one should never attempt modifications to existing firearms or scratch building of weapons without a thorough assessment of the proposed design, considerable experience, and/or the supervision of a competent gunsmith. Additionally, contemporary U.S. firearms law is a truly mind boggling amalgam of disjointed regulations and rulings of which one can readily and innocently run afoul. As a result, I resolved to design and execute the FrankenSTEN in such a manner that all the pressure-bearing surfaces were comprised of factory components retaining factory tolerances and, further, to build the gun around my registered M16 receiver to ensure compliance with the National Firearms Act.

Just what is a FrankenSTEN?

The FrankenSTEN is a newly constructed 9mm upper receiver that mounts on an M16 lower receiver and allows selective-fire from a side-mounted 32-round STEN magazine. The original STEN uses thin-walled steel tubing with about a 1.5-inch internal diameter as its receiver stock while the FrankenSTEN upper receiver is built from heavy-walled tubing of about 1-inch internal diameter to incorporate the smaller-diameter M16 recoil system. The FrankenSTEN utilizes the factory barrel, magazine housing, magazine, and ejector from a Mark III STEN gun as well as a highly modified STEN gun bolt. The bolt requires the most redesign and machine work as the original STEN is an open-bolt gun with a fixed firing pin whereas the FrankenSTEN utilizes a closed bolt with a floating firing pin and the hammer and trigger assembly from the M16 lower receiver.

For this project I ordered a pair of Mark III STEN kits just so there would not be a shortage of spare parts. Fortuitously, one of the kits came with a bronze Mark IIS bolt originally designed for the sound-suppressed Mark II STEN. Original steel STEN bolts are quite hard and the bronze bolt made the lathe and mill work much more palatable without sacrificing structural integrity.

The conversion of the bolt involves grinding the fixed firing pin flat with the breech face, boring out the core of the bolt to accommodate the floating firing pin, and cutting the appropriate slots to allow clearance for the hammer. The resulting assembly must then be turned down to the diameter of the new receiver on a lathe and the key affixed to the top with Allen screws. Additionally, the FrankenSTEN bolt requires a minor redesign to support the ejector spring once the floating firing pin is installed. Once the bolt is complete metal is carefully removed from the rear of the hammer clearance slot to time the auto safety sear. The sear should trip the hammer just as the bolt is fully in battery. Removing too little material and tripping the sear early can result in a blown primer or a cartridge detonating out of battery. Removing too much prevents the sear from tripping at all. An original M16 bolt carrier serves as an excellent template for this operation as its dimensions are obviously designed to accomplish the same end.

The upper receiver itself is cut from thick-walled tube stock and has the two receiver mounting lugs welded to the bottom as well as an MP5-style cocking tube mounted over the top. A small piece of sheet steel stock is cut to size and welded between the new receiver and the magazine well to give the magazine well and its ejector, the dimensions of which are designed around the wider tubing, the appropriate standoff from the reciprocating bolt.

I am convinced that a determined hobbyist with a limitless supply of steel stock and fiber-reinforced cutoff wheels for his Dremel grinder could build a functioning Abrams tank if he had the time to put into the project. For this undertaking, however, I did use a compact hobby-size combination milling machine and metal lathe that I have in my workshop at home. However, the tool work required by this project is well within the capabilities of even the most primitive commercial machine shop.

An Elegant Little Machine

As can be seen in the accompanying function diagram, the FrankenSTEN operates on a straight blowback principle relying on the mass of the reciprocating parts and the strength of the recoil spring to counteract the recoil impulse of the cartridge. In the case of the FrankenSTEN, the weapon’s bolt moves rearward with the detonation of the cartridge and impinges upon the buffer and recoil spring of the M16 lower receiver. The upper is designed such that the key affixed to the top of the bolt strikes the steel cap in the back of the cocking tube just a couple of millimeters before the bolt would otherwise impact the receiver ring on the M16 lower. A small rubber buffer is incorporated into the end cap to attenuate the violence of this impact. In this way the not insubstantial recoil impulse from the straight blowback 9mm cartridge is retained within the upper assembly and does not risk damaging an irreplaceable transferable lower.
All this conspires to give the FrankenSTEN a rather unconventional firing sequence. At the moment of ignition the recoil impulse of the fired cartridge pushes the bolt back against the spring-loaded M16 buffer. The rearward movement of the bolt is arrested at a fixed point by the rubber buffer and end cap assembly at the rear of the cocking tube. Inertia carries the buffer back another 3/4-inch or so against spring compression until the recoil spring fully absorbs the impact and returns the buffer forward. The buffer then catches the bolt and shoves the entire assembly forward toward the breech face. Three quarters of an inch from the breech face the bolt key contacts the forward spring in the cocking tube, which also serves to keep the cocking handle in the forward position during cycling, and slides into battery ready to fire another round. The combination of the forward spring and the rearward rubber buffer serves to yield an extraordinarily smooth action on firing.

The MP5-style cocking tube mounted on the top of the upper receiver serves to contain the reciprocating parts as described above as well as raise the sights to a point where the average shooter can access them. The cocking tube component of the design gives the gun a nice aesthetic symmetry but it does entail some fairly tedious welding and involves another couple of moving parts and their commensurate possibilities for failure. As such, the hobbyist could undertake a similar project by incorporating a simple reciprocating charging handle mounted to the bolt as with the original STEN and greatly simplify fabrication.

The rear cap is retained via a 1/4-inch pin with a ball detent picked up from my local Ace hardware that makes disassembly a snap without tools. The foreword handguard is laminated together from wood strips, carved to shape, and wrapped in non-skid rubber flooring material. The non-skid material is affixed to the handguard with JB Weld automotive epoxy and the entire assembly is retained in place by the accessory rail mounted to the right side of the receiver with Allen screws. This handguard design facilitates a beefy grip and effectively dissipates the heat of extended firing sessions. The accessory rail mounts either a laser sight or a tactical flashlight and sits at the two o’clock position, out of the way of the weak hand when fired right-handed.

Balancing a blowback weapon is a relatively delicate affair and the FrankenSTEN’s design lends itself to tinkering. For reliable functioning the mass of the bolt and strength of the recoil spring must be balanced to the recoil impulse of the cartridge to produce reliability at the desired rate of fire. Too much mass or too stout a spring and the cartridge won’t cycle the action. Too little and the recoil impulse can damage the reciprocating parts or allow the cartridge to fire out of battery. In this case I incorporated a steel weight into the rear end of the modified STEN bolt to increase its mass and carefully selected the spring that rests between the cocking handle and the bolt in the cocking tube. By altering this spring one can increase or decrease the force pushing rearward against the weapon’s recoil system, in essence boosting the recoil energy available to cycle the action. This feature allows the fine tweaking necessary to achieve an optimal rate of fire. The original STEN bolt weighs 20.75 ounces versus the sum of the FrankenSTEN’s bolt and buffer of 13 ounces. The M16 buffer spring, however, is a great deal stiffer and stronger than its British counterpart and helps offset the weapon’s brisk recoil impulse.

It is Alive!

Enough about the engineering of the project, the proof of a weapon’s design is discovered at the range rather than the drafting table. As for the FrankenSTEN, the first several proof rounds were fired off of a mechanical rest from a safe distance. While the bolt face and chamber dimensions were retained exactly as from the original STEN, even pistol cartridge chamber pressures are extraordinary and I had no interest in having a piece of ruptured cartridge casing ruin my striking good looks. Though it was more than a bit unexpected, the gun ran flawlessly from the get-go. The action is smooth and tight on semiautomatic owing to the closed-bolt design and inherently nice M16 trigger. On full-auto the rate of fire floats around 800 to 900 rpm. This is a good bit faster than the original STEN gun but not nearly the ammo gobbling buzz of a MAC-10. As a result, two- and three-round bursts are painless to the experienced trigger finger and accuracy is not too bad considering the design incorporates a fifty-year-old pistol-length military-surplus barrel. Despite the smooth action, the recoil impulse is surprisingly sharp and the noise of the reciprocating spring in the M16 buttstock is striking, even through earplugs. Amazingly, there were no failures to feed during my first outing to the range with the FrankenSTEN, a tribute to the robust nature of the original design. Overall, the impression upon firing the FrankenSTEN is much more akin to the butter-smooth cycling of an MP-5 than the bone-jarring clunkiness of a STEN.

The sights are still a little low for my tastes and, had I been doing it over again, I would probably utilize a fixed charging handle on the bolt and an elevated carrying handle on which to mount the sights or optics. This minor impediment notwithstanding, the gun is a real head-turner at the range and is still quite a buzz to shoot. The thick-walled receiver tubing gives the weapon a decidedly solid feel, not unlike an Uzi, and the gun is a pleasure to keep on target. The entire package is compact and the side-mounted magazine, while lopsided when fully loaded, gives the piece a nice low profile when firing prone or from a bench.

All in all, I am extremely pleased with my FrankenSTEN. The project kept me out of trouble in the evenings for a couple of months and promises to give years of reliable service at the range. Magazines can still be had for about $3 each and I have already amassed a ridiculous quantity. The most amazing thing about this project, however, is that even if one includes the price of the combination milling machine/lathe, my inexpensive arc welder, the two Mark III STEN kits, and the raw steel stock I still have less invested in the FrankenSTEn than it would take to buy a Colt 9mm conversion for my registered M16. All this and my gun collection now sports something truly unique. So long as a project of this sort is approached with the appropriate respect and adherence to safety principles, it is well within the capabilities of the average American basement tinker with the proper tools. It also provides the tiniest taste of what the good ole days were like back before 1986. I sure do miss them.